The present invention relates to an apparatus and a method for controlling an internal combustion engine that has a fuel vapor treating apparatus, which collects fuel vapor in a fuel tank to a canister without releasing the fuel vapor into the atmosphere and purges the collected fuel vapor to the intake passage of the engine as necessary.
A typical internal combustion engine driven with volatile liquid fuel includes a fuel vapor treating apparatus. The fuel vapor treating apparatus has a canister for temporarily storing fuel vapor generated in a fuel tank. When necessary, fuel vapor collected by an adsorbent in the canister is purged to the intake passage of the engine from the canister through a purge passage, and is mixed with air drawn into the engine. The fuel vapor is combusted in the combustion chamber of the engine together with fuel injected from the injector. A purge control valve located in the purge passage adjusts the flow rate of gas (purge gas) containing fuel vapor to the intake passage.
In the above internal combustion engine, the air-fuel ratio of combustible gas mixture supplied to the combustion chamber is detected. The amount of fuel injected from the injector is controlled such that the detected actual air-fuel ratio matches with a target value. To optimally control the air-fuel, the amount of fuel injected from the injector needs to be controlled by taking the amount of fuel vapor purged to the intake passage through the purge passage into consideration.
Typically, the amount of injected fuel is controlled in the following manner when the influence of fuel vapor is taken into consideration. First, a basic fuel injection amount (time) is computed based on parameters indicating the running state of the engine, such as the engine speed and the intake flow rate. Then, a final fuel injection amount (time) is determined by adjusting the basic fuel injection amount with a air-fuel ratio feedback correction factor, an air-fuel ratio learning value, a purging air-fuel ratio correction factor, and correction factors obtained based on the running states. The air-fuel ratio feedback correction factor corresponds to the difference between the air-fuel ratio of the previous fuel injection relative and the stoichiometric air-fuel ratio. The air-fuel ratio feedback correction factor is used for permitting the air-fuel ratio in the current fuel injection to approximate the stoichiometric air-fuel ratio. The air-fuel ratio learning value is a correction factor that is learned and stored for each running state region based on the results of air-fuel ratio feedback control in different running state regions. Using the air-fuel ratio learning value improves the accuracy of the air-fuel ratio feedback control.
The purge air-fuel ratio correction factor is obtained by considering the influence of the fuel vapor introduced into the intake passage to the air-fuel ratio. The purge air-fuel ratio correction factor is computed based on a purge ratio and a vapor concentration learning value. The purge ratio refers to a coefficient that represents the ratio of the flow rate of purge gas introduced into the intake passage to the flow rate of intake air in the intake passage. The vapor concentration learning value refers to a coefficient that reflects the concentration of the vapor component in the purge gas. The product of the purge ratio and the vapor concentration learning value is used as the purge air-fuel ratio correction factor for correcting the air-fuel ratio.
When the air-fuel ratio deviates from a target air-fuel ratio while fuel vapor is being purged, the vapor concentration learning value, which is used for computing a purging air-fuel ratio correction factor, is renewed. At this time, if the vapor concentration learning value is renewed by a certain amount that has been determined regardless of the purge ratio, the air-fuel ratio is deviated from the target air-fuel ratio particularly when the purge ratio changes from a smaller value to a greater value.
That is, the air fuel ratio of an internal combustion engine is fluctuated not only by the influence of purging, but also by changes in the running state of the vehicle. Therefore, if the deviation of the air-fuel ratio is entirely reflected on the renew amount of the vapor concentration learning value on the assumption that deviation of the air-fuel ratio is entirely caused by the influence of the purging, the computed vapor concentration learning value is deviated from the actual vapor concentration. When the purge ratio is not changing or small, deviation of the vapor concentration learning value from the actual vapor concentration causes no drawbacks. However, when the purge ratio changes from a smaller value to a greater value, deviation of the vapor concentration learning value causes a problem.
For example, suppose that the air-fuel ratio is deviated from a target air-fuel ratio by 2% due to changes in the running state of the vehicle, not due to the influence of purging, and that the purge ratio is small, for example, 0.5%. At this time, if the deviation of the air-fuel ratio is entirely reflected on the renew amount of the vapor concentration learning value on the assumption that the deviation of the air-fuel ratio is entirely caused by the influence of the purging, the computed vapor concentration learning value is deviated from the actual vapor concentration by 4% per unit purge ratio (4%=2%/0.5%). In this case, if the purge ratio is maintained at 0.5%, the computed vapor concentration learning value continues to be different from the actual vapor concentration by 4%.
However, if the purge ratio is increased from 0.5% to 5%, the deviation of the computed vapor concentration learning value will be 20% (20%=4% (deviation per unit purge ratio)xc3x97purge ratio 5%). When the deviation of the computed vapor concentration learning value is 20%, a fuel injection amount corrected based on the computed vapor concentration learning value is significantly deviated from a fuel injection amount required for maintaining the target air-fuel ratio. Accordingly, the air-fuel ratio is significantly deviated from the target air-fuel ratio.
On the other hand, if the air-fuel ratio is deviated from a target air-fuel ratio by 2% due to the influence of the running state of the vehicle, and the purge ratio is a great value, for example 5%, the computed vapor concentration learning value is only 0.4% per unit purge ratio (0.4%=2%/5%). Therefore, the errors of the vapor concentration learning value are insignificant. Also, when the purge ratio falls from a great value, the deviation of the vapor concentration learning value is gradually decreased, which causes no particular drawbacks. That is, problems are caused by renewal of the vapor concentration learning value while the purge ratio is low.
To solve such problems, Japanese Laid-Open Patent Publication No. 10-227242, for example, discloses an art in which, when a vapor concentration learning value is renewed, the renew amount is set to a smaller value if a purge ratio is a small value compared to a case where the purge ratio is a great value. This prevents an erroneous learning of the vapor concentration due to a deviation of the air-fuel ration caused by the influence of the running state of a vehicle.
As described above, a purge ratio is a theoretical ratio of the flow rate of purge gas introduced to an intake passage to the flow rate of intake air flowing through the intake passage. A small value of the purge ratio represents that the flow rate of purge gas is small relative to the flow rate of intake air. Therefore, when the intake air flow rate is increased and the intake negative pressure acting on the intake passage is decreased (or when the intake pressure is increased), the purge ratio has a small value. The purge gas flow rate is also changed according to the intake pressure acting o the intake passage. Since the pressure loss in the intake negative pressure varies for each internal combustion engine, the purge gas flow rate varies for each internal combustion engine if the intake negative pressure and the purge ratio are both small. However, the method disclosed in the above publication simply sets the renew amount of a vapor concentration learning value to a small value when the purge ratio is small, but does not take variations of the purge gas flow rate into consideration. This method can cause an erroneous learning of the vapor concentration. Accordingly, the concentration of fuel vapor is not accurately obtained when the purge ratio is small. This results in an inaccurate computation of fuel injection amount, and lowers the accuracy of the air-fuel ratio control.
Accordingly, it is an object of the present invention to provide an apparatus and a method for controlling an internal combustion engine, in which apparatus and method a vapor concentration is learned in a favorable manner and the accuracy of an air-fuel ratio control is improved.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an apparatus for controlling the air-fuel ratio of air-fuel mixture drawn into a combustion chamber of an engine is provided. An intake passage of the engine is connected to a canister, which adsorbs fuel vapor generated in a fuel tank. Gas containing fuel vapor is purged as purge gas from the canister to the intake passage through a purge control device by intake negative pressure generated in the intake passage. The apparatus includes a computer and a sensor for detecting the air-fuel ratio of the air-fuel mixture. According to a deviation of a detected air-fuel ratio relative to a target air-fuel ratio, the computer renews a vapor concentration value representing the concentration of fuel vapor contained in the purge gas by a predetermined renew amount at a time. The computer sets the amount of fuel supplied to the combustion chamber according to the renewed vapor concentration value such that the detected air-fuel ratio seeks the target air-fuel ratio. The computer sets a smaller value of the renew amount for a greater value of the load on the engine.
The present invention also provides a method for controlling the air-fuel ratio of air-fuel mixture drawn into a combustion chamber of an engine. An intake passage of the engine is connected to a canister, which adsorbs fuel vapor generated in a fuel tank. Gas containing fuel vapor is purged as purge gas from the canister to the intake passage through a purge control device by intake negative pressure generated in the intake passage. The method includes: detecting the air-fuel ratio of the air-fuel mixture; renewing a vapor concentration value representing the concentration of fuel vapor contained in the purge gas by a predetermined renew amount at a time according to a deviation of a detected air-fuel ratio relative to a target air-fuel ratio; setting the amount of fuel supplied to the combustion chamber according to the renewed vapor concentration value such that the detected air-fuel ratio seeks the target air-fuel ratio; and setting a smaller value of the renew amount for a greater value of the load on the engine.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.